Aircraft Drawings

The airframe of a fixed-wing aircraft is generally considered to consist of five principal units, the fuselage, wings, stabilizers, flight control surfaces, and landing gear. Helicopter airframes consist of the fuselage, main rotor and related gearbox, tail rotor (on helicopters with a single main rotor), and the landing gear. The airframe components are constructed from a wide variety of materials and are joined by rivets, bolts, screws, and welding or adhesives. The air· craft components are composed of various parts called structural members (i.e., stringers, longerons, ribs, bulkheads, etc.). Aircraft structural members are designed to carry a load or to resist stress. A single member of the structure may be subjected to a combination of stresses. In most cases the structural members are designed to carry end loads rather than side loads: that is, to be subjected to tension or compression rather than bending. Strength may be the principal requirement in cer· tain structures, while others need entirely different qualities. For example, cowling, fairing, and simi· lar parts usually are not required to carry the stresses imposed by flight or the landing loads. However, these parts must have such properties as neat appearance and streamlined shapes.

Major Structural Stresses

In designing an aircraft, every square inch of wing and fuselage, every rib, spar, and even each metal fitting must be considered in relation to the physical characteristics of the metal of which it is made. Every part of the aircraft must be planned to carry the load to be imposed upon it. The determi· nation of such loads is called stress analysis. AI· though planning the design is not the function of the aviation mechanic, it is, nevertheless, important that he understand and appreciate the stresses involved in order to avoid changes in the original design through improper repairs.

There are five major stresses to which all aircraft are subjected (figure 1-1):

(1) Tension.

(2) Compression.

(3) Torsion.

(4) Shear.

(5) Bending.

The term "stress" is often used interchangeably with the word "strain." Stress is an internal force of a substance which opposes or resists deformation. Strain is the deformation of a material or substance. Stress, the internal force, can cause strain.

Tension (figure 1-1a) is the stress that resists a force that tends to pull apart. The engine pulls the aircraft forward, but air resistance tries to hold it back. The result is tension, which tries to stretch the aircraft. The tensile strength of a material is measured in p.s.i. (pounds per square inch) and is calculated by dividing the load (in pounds) required to pull the material apart by its cross-sectional area (in square inches).

Compression (figure 1-1b) is the stress that resists a crushing force. The compressive strength of a material is also measured in psi. Compression is the stress that tends to shorten or squeeze aircraft parts.

Torsion is the stress that produces twisting (figure 1-1c). While moving the aircraft forward, the engine also tends to twist it to one side, but other aircraft components hold it on course. Thus, torsion is created. The torsional strength of a material is its resistance to twisting or torque.

Shear is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two riveted plates in tension (figure 1-1d) subject the rivets to a shearing force. Usually, the shearing strength of a material is either equal to or less than its tensile or compressive strength. Aircraft parts, especially screws, bolts, and rivets, are often subject to a shearing force.

Bending stress is a combination of compression and tension. The rod in figure 1-1e has been shortened (compressed) on the inside of the bend and stretched on the outside of the bend.

Fixed-Wing Aircraft

The principal components of a single-engine, propeller-driven aircraft are shown in figure 1-2. Figure 1-3 illustrates the structural components of a typical turbine powered aircraft. One wing and the empennage assemblies are shown exploded into the many components which, when assembled, form major structural units.

Fuselage

The fuselage is the main structure or body of the aircraft. It provides space for cargo, controls, accessories, passengers, and other equipment. In single engine aircraft, it also houses the powerplant. In 2 multi-engine aircraft the engine may either be in the fuselage, attached to the fuselage, or suspended from the wing structure. They vary principally in size and arrangement of the different compartments. There are two general types of fuselage construction, the truss type, and the monocoque type. A truss is a rigid framework made up of members such as beams, struts, and bars to resist deformation by applied loads. The truss-framed fuselage is generally covered with fabric.

Truss Type

The truss type fuselage frame (figure 1-4) is usually constructed of steel tubing welded together in such a manner that all members of the truss can carry both tension and compression loads. In some aircraft, principally the light, single-engine models, truss fuselage frames are col18tructed of aluminum alloy and may be riveted or bolted into one piece, with cross-bracing achieved by using solid rods or tubes.